1. Field of the Invention
The present invention relates to differential analytical techniques for determining the composition, phase, structure, or other properties of a sample of material.
2. Background of the Invention
Thermal analysis techniques generally comprise measuring a physical parameter as a function of the temperature of the sample. The sample temperature is strictly controlled throughout the analysis. Whenever the sample undergoes a chemical transformation, a physical transformation, a phase change or another transition which affects the physical parameter being measured, the changes in that physical parameter may be interpreted to analyze the composition, structure, or thermal stability of the sample.
In differential thermal analysis techniques, the physical parameter of the sample being measured is compared to that of a reference, as a function of the temperature of the sample. The difference in the physical parameter measured for the sample and that measured for the reference is then recorded. The differential thermal analysis technique compensates for the effects of heating rate and ambient conditions that could cause changes in the measured physical parameter of the sample and reference. The differential thermal analysis technique can increase the sensitivity of the measurement of the physical parameter by removing large offsets in the value of the physical parameter whenever the precision of the measuring apparatus is limited.
One common thermal analysis technique is Differential Scanning Calorimetry (DSC). DSC is a thermal analysis technique which measures the temperatures and heat flow associated with chemical or physical transitions in materials as a function of time and temperature. The classical DSC method comprises heating the sample material and the reference material at a constant rate of temperature increase, typically 5.degree. C. to 20.degree. C. per minute, and recording the difference in heat flow into or out of the sample and the reference as a function of temperature.
Other differential thermal analysis techniques include Pressure Differential Scanning Calorimetry (PDSC), Differential Thermal Analysis (DTA), Pressure Differential Thermal Analysis (PDTA), Differential Photocalorimetry (DPC), pressure Differential Photocalorimetry (PDPC), Differential Thermogravimetry (DTG), and Pressure Differential Thermogravimetry (PDTG).
Furthermore, the present invention applies to all differential analysis techniques. It is not limited to differential thermal techniques. Whereas thermal differential analysis techniques depend upon temperature as the driving variable, non-thermal differential analysis techniques depend upon another driving variable, such as pressure, applied stress, or wavelength of incident radiation.
DSC measurements provide quantitative and qualitative information about the sample transitions that involve endothermic or exothermic processes, or changes in heat capacity. Pressure DSC is a related technique in which the heat flow and temperature of transitions are measured as a function of temperature under controlled pressure, or as a function of pressure under controlled temperature.
Differential Thermal Analysis, like DSC, measures the temperatures and heat flow associated with transitions in materials as a function of time and temperature. However, unlike DSC, DTA results are semi-quantitative. Pressure DTA is a related technique in which the heat flow and temperature of transitions are measured as a function of temperature under controlled pressure, or as a function of pressure under controlled temperature. DTA is generally carried out at higher temperatures than DSC.
Differential Photocalorimetry measures the heat absorbed or released by a sample as it and a reference are exposed simultaneously to radiation of known wavelength and intensity. Pressure DPC is a related technique in which the heat absorbed or released by the sample is measured as a function of temperature under controlled pressure, or as a function of pressure under controlled temperature.
Differential Thermogravimetry measures the differential weight change of a sample and a reference as a function of time and temperature. Pressure DTG is a related technique in which the differential weight change is measured as a function of temperature under controlled pressure, or as a function of pressure under controlled temperature.
High resolution analytical techniques are described in U.S. patent application Ser. No. 07/638,847. That application is incorporated by reference herein. Those techniques seek to improve the resolution of changes in a characterizing physical parameter by controlling the rate of sample heating during transitions as a function of the rate of change of the physical parameter. When non-differential thermal analysis techniques are used, the high resolution techniques are effective in improving resolution for many transitions. However, they usually reduce the sensitivity of transitions when applied to differential thermal analysis techniques. This is because, for most differential thermal analysis techniques, the magnitude of the differential physical parameter is a direct function of the heating rate. Reducing the heating rate during transitions causes the differential signal to change, which may alter or obscure the true differential signal resulting from the transition event. This obscuring of the physical parameter can reduce the utility of the high resolution techniques when applied to conventional differential thermal analysis techniques.
Conventional differential thermal analysis techniques are limited in their ability to separate non-reversible events caused by enthalpic processes (chemical or physical) from reversible events such as changes in the heat capacity of the sample. This is because the reversible and non-reversible processes often occur simultaneously, or occur severely overlapped in time and/or temperature.
In addition, conventional and high resolution thermal analysis techniques cannot distinguish between rapidly reversible and non-rapidly reversible transitions within a single heating or cooling scan of the sample.